1. Field of the Invention
The present invention relates to a flash exposure apparatus, and more particularly, it relates to a flash exposure apparatus which can perform pre-emission to avoid a red-eye effect.
2. Related Background Art
Red-eye is an effect in which the pupil of a human eye has a reddish (or golden) color in a photograph obtained by the use of an electronic flash device. This red-eye effect occurs when flash light emitted by the electronic flash device is reflected by the retina of the eye and is imaged on a film. Since there are many capillary vessels in the retina and hemoglobin in the blood is red, the light reflected by the retina appears red.
It has been found experimentally that the red-eye effect is most prominent under two conditions: 1) when a flash photograph of a person is taken in a dark environment, and 2) when the distance between the emission portion of an electronic flash device and an optical axis of a photo-taking ("taking" lens) is short.
In condition 1) the diameter of the pupil of the eye may increase to 7-8 mm, increasing the amount of light incident on the eye and the amount of light reflected from the retina. With regard to condition 2), the retina of the eye has a high reflection factor and a high degree of orientation of reflected light.
Accordingly, when the emission portion of the electronic flash device is situated in the vicinity of the optical axis of the taking lens, and, thus, three elements (emission portion, taking lens and eye) are arranged in such a positional relation that the reflected light regularly reflected by the retina is introduced into the taking lens, a strong red-eye effect will occur. That is to say, when the angle formed between a line extending from the taking lens to the pupil of the eye (an object) and a line extending from the emission portion of the flash source to the pupil is smaller than a predetermined value, the red-eye effect occurs without fail. Experimentally, it was found that such predetermined value of the angle is about 2 to 2.5 degrees. Accordingly, it is possible to avoid the red-eye effect by keeping the emission portion of the electronic flash device away from the optical axis of the taking lens. However, there is a practical limit to distance between the emission portion and the optical axis of the taking lens. Even when the emission portion is separated from the optical axis of the taking lens by the distance limit, if the distance between the taking lens and the object (referred to as "object distance" hereinafter) is lengthened, the above-mentioned angle tends to be lower than the above-mentioned predetermined value. In other words, when the object distance has a predetermined value or more, it will be difficult to avoid the red-eye effect.
For these reasons, various techniques for avoiding the red-eye effect have been proposed. For example, a magazine "psa JOURNAL" (July, 1952) discloses a method in which human eyes are exposed to bright light to decrease the diameter of the pupil of the eye to 3 mm or less, and then the object (including such eyes) is photo-taken by energizing the flash device, thus avoiding the red-eye effect. Further, the Japanese Patent Publication No. 58-48088 discloses a technique in which, before taking a photograph, a pre-emission by means of a preliminary emission lamp is executed for a time required for closing the pupil, and when the pupil substantially reaches its minimum diameter the emission portion of the electronic flash device is energized to take a photograph. In addition, the Japanese Laid-Open Patent Application No. 58-9130 discloses a technique in which two flash discharge tubes are provided, and, after the pre-emission is executed by one of the flash discharge tubes to close the pupil of the eye, main-emission is executed by the other flash discharge tube to take a photograph.
FIG. 8 shows an example of a conventional flash exposure apparatus for avoiding the red-eye effect. In operation, capacitors band 7 are charged at about 300 volts by a voltage booster circuit 1. In accordance with a release signal from a camera (not shown), a thyristor 9 is firstly turned ON, whereby high voltage is applied to a trigger electrode of a Xe tube 5, and the Xe tube 5 is illuminated by the energy accumulated in the capacitor 6. Then, when a shutter is opened, a switch SW1 is turned ON. In response to this, the thyristors 9 and 10 are simultaneously turned ON through terminals a-c and b-d, whereby the Xe tube 5 is illuminated by the energy accumulated in the capacitor 7.
A control circuit 100 shown in FIG. 8 functions to generate strobe emission (electronic flash emission) twice. In the first strobe emission, the irises of the eyes of the person to be photographed (object) are closed or restricted before taking a photograph, and, after the irises are closed, the second strobe emission is performed, thereby to avoid the red-eye effect.
However, with the construction of the flash exposure apparatus shown in FIG. 8, two charging capacitors must be provided in a camera body which should be compact and light. Thus, naturally, the capacity or ability of the capacitor for the flash device cannot be large, thereby reducing maximum amount of light which can be used in each exposure (i.e., decreasing the guide number). As a result of the reduction of the guide number, a problem arises in that freedom of exposure using the flash device, for example, the range of selection of lenses having different focal length, is restricted.
In the above-mentioned conventional flash exposure apparatus, the pre-emission having constant intensity of light is generated for a given time. Accordingly, even if the amount of light is quite enough to avoid the red-eye effect when the object (person) is positioned near the camera, when the object is positioned far from the camera (in which case the red-eye effect is apt to occur), the amount of light will be insufficient, with the result that the red-eye effect cannot be fully avoided.